qemu: dump: Report better error when dumping VM with passthrough devices

For the regular dump operation we migrate the VM to a file. This won't
work when the VM has passthrough devices assigned. Rather than reporting
a cryptic error from qemu run our check whether it can be migrated.

This does not influence the memory-only dump that is allowed with
passthrough devices.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=874418

src/qemu/qemu_driver.c

@@ -3481,6 +3481,10 @@ doCoreDump(virQEMUDriverPtr driver,
                              "memory-only dump"));
             goto cleanup;
         }
+
+        if (!qemuMigrationIsAllowed(driver, vm, vm->def, false, false))
+            goto cleanup;
+
         ret = qemuMigrationToFile(driver, vm, fd, 0, path,
                                   qemuCompressProgramName(compress), false,
                                   QEMU_ASYNC_JOB_DUMP);